A wide variety of circuit applications require reliable generation of bilevel state output signals in response to input conditions. One example is a controller that must allow a circuit board to be safely inserted into and removed from a live backplane. When a circuit board is inserted into a live backplane, large bypass capacitors on the board can draw huge inrush currents from the backplane power bus as they charge. The high current can cause the backplane voltage to dip and can destroy the board's bypass capacitors, metal traces or connector pins. Undervoltage detection and responsive circuit disconnect regulation with precise under-voltage hysteresis is essential to prevent circuit fault damage.
A circuit 10, currently used for undervoltage protection, is shown in FIG. 1. Power supply voltage supply VIN is applied to an input node of a resistive voltage divider circuit comprising series connected resistors 12, 14 and 16. A junction point or node between resistors 12 and 14 is indicated by INH. A junction point or node between resistors 14 and 16 is indicated by INL. Normally open switch contact 18 is connected between INH and a negative input of comparator 20. Normally closed contact 19 is connected between INL and the negative input of the comparator. For explanatory purposes only, voltages VEXT and VINH, which respectively represent the voltages across resistors 14 and 12, are illustrated. A reference voltage REF is connected to the positive input of the comparator. The output of comparator 20 is applied to a relay or the like, not shown, that simultaneously controls operation of both switch contacts in response to the comparator output. While a well-known relay and switch are exemplified, this arrangement may represent known equivalent electronic switches and control circuits therefor.
The comparator output produces a bilevel state signal that can function as a switch signal to a controller connected thereto. A high level state of the comparator output indicates that the supply voltage VIN has decreased to an undesirable level. In response to this state, a controller can initiate appropriate disconnect protection. During normal supply voltage conditions, the comparator output assumes a low level state.
During normal level voltage supply conditions, as illustrated, switch 19 connects INL to the negative comparator input while switch 18 is open. As INL voltage is greater than REF, the comparator output is at a low level state. If VIN decreases, INL decreases proportionately. When INL voltage decreases to REF, the level at which VIN reaches an undesirable level, the comparator output changes to a high state, effecting a turn-off signal. In response to the high level comparator output, the comparator input switch contacts are activated to open switch contact 19 and close switch contact 18. At this time, the negative input of the comparator is connected to INH. The voltage VINH is a smaller fraction of VIN than the voltage at INL, and thus lower than, the INL voltage. As a result of the switch actuation, the voltage at the negative input of the comparator has decreased by VEXT, the voltage across resistor 14. The comparator will continue to output a high level state signal until the voltage at VINH rises to REF to change the output back to the low level state. At this time, the voltage at INL exceeds REF by the voltage across resistor 14. The voltage of VIN is higher than the VIN undervoltage threshold at which the comparator is activated to a high state output.
The circuit thus provides a hysteresis switching function. A high output is produced when the supply voltage decreases to a low voltage threshold and a low output is produced when the supply voltage exceeds a threshold that is higher than the low voltage threshold. That is, even if the supply voltage is increased from below the low voltage threshold, the output will not change state until the supply voltage increases further by the hysteresis value. The hysteresis value corresponds to VEXT, which is determined by the voltage divider circuit, particularly the value of resistor 14. Hysteresis can be adjusted externally by changing the value of resistor 14. The provision of hysteresis limits oscillation of the output signal between states when the supply voltage is close to the low threshold level.
A disadvantage of the circuit just described is that the voltage taps INL and INH can be inadvertently misconnected to the comparator input such that switch 18 is connected to INL and switch 19 is connected to INH. With such interconnections, the negative comparator input is coupled through switch 19 to INH during nominal VIN levels. As VINH exceeds REF, the comparator output is low. The output remains at the low state until VINH decreases to the threshold REF. The comparator changes output states to a high level, which then activates the switch to open contact 19 and close contact 18. As the resistor divider circuit has been improperly connected, the negative comparator input is now connected to INL. As the voltage at INL at this time exceeds VINH and REF by VEXT, the comparator again changes states to the low level resulting again in a change in switch contact states. Operation of the circuit becomes unstable with continuous oscillation of the states of the comparator output.
The need thus exists for a comparator circuit that has precise default hysteresis that can also be adjusted externally without risk of oscillation.